The three specific aims of this study are 1) to assess the impact of iron status on relative differences in absorption of heme and non-heme iron among pregnant women and non-pregnant women, 2) to assess the magnitude and the determinants of heme and non-heme iron transfer to the fetus over the last trimester of pregnancy, and 3) to characterize relationships between placental iron binding proteins with the enrichment of stable iron isotopes in the neonate at birth. The investigators hypothesize that there will be up regulation of heme and non-heme iron absorption and up regulation of placental iron transport proteins in response to low maternal iron stores.